Optical networks are continuing their exponential capacity growth. Most recent 2017 projections show worldwide fixed internet traffic growing at 23% Compound Annual Growth Rate (CAGR), while mobile traffic which starts at a lower absolute level is projected to grow at 46% CAGR over the next 5 years. Unfortunately for network operators, their revenues are only seeing very modest growth rates. Such bandwidth growth rates are straining network and equipment designs, and can only be realized by combining substantial network architecture changes with more efficient and lower cost equipment designs. Looking at the details of the forces driving bandwidth demands, the dominant component is consumer video at just under 80% of the total and is expected to grow in importance. Further, peak bandwidth is growing faster than average, indicating that consumers are becoming an even more dominant component during specific times of the day. These trends are forcing network architecture evolution to put an increasing amount of content closer to the user at the network edge. This relieves some bandwidth burden in the network core, limits the distance that content has to traverse over networks, and allows substantial amounts of content replication to occur during bandwidth consumption lulls improving overall network utilization efficiency.
These trends increase the importance of optimized high-capacity optical transport designs over shorter distances. High capacity, low power optical links are widely used inside data centers over reaches <2 km and have seen recent scaling from 40 Gbps to 100 Gbps, with development work addressing 400 Gbps designs. Similarly, power and footprint limits placed on individual data center buildings are requiring high capacity low power optical interconnects within data center campus type deployments, with optical reaches on the order of <40 km. The expansion of mobile networks with planned evolution to 5G architectures will require optical links in the <20 km range for front-haul and <80 km range for backhaul type links. Telecom carrier efforts in Central Office Rearchitected as Data Center (CORD) is introducing a similar content processing model more widely across the networks, with the expected desire in high-capacity low power optical links with <80 km reach.
Also, optical transceivers are being standardized to support multiple vendor interoperability through Multisource Agreements (MSAs). Exemplary MSAs for 40G, 100G, 200G, and 400G include C Form-factor Pluggable (CFP) and variants thereof (e.g., CFP2, CFP4, CXP), Quad Small Form-factor Pluggable (QSFP) and variants thereof (e.g., QSFP+, QSFP2, QSFP28, QSFP-DD (Double Density), etc.), Octal Small Form-factor Pluggable (OSFP), and the like. Each MSA defines the transceiver's mechanical characteristics, management interfaces, electrical characteristics, and thermal requirements. Power consumption is becoming a critical factor and is starting to outweigh other considerations in many designs, either directly or indirectly by defining packaging density, cooling complexity, and cost, etc. In particular, transceivers with a target optical reach of <100 km are becoming more critical, and have a requirement to fit into data center pluggable sockets sized for short-reach client interconnect, i.e., MSAs. For example, QSFP28 targets 100 G at <4 W, QSFP-DD targets 200 G and 400 G at <7 W, and OSFP targets 200 G and 400 G at <15 W.
There are a number of approaches that have been explored in academia and industry to address high capacity optical links with approximately 80 km single-hop reach. An example product uses a 28 Gbaud Pulse Amplitude Modulation-4 (PAM4) modulation to provide 80 km reach, with optical dispersion compensation. More sophisticated approaches with more complex DSP are outlined in D. Plant et al., “Optical communication systems for datacenter networks,” OFC 2017, Tutorial W3B.1. It is worthwhile to note four approaches that provide a good representation of the current state of the art. A first approach is described in N. Eiselt et al., “First Real-Time 400 G PAM-4 Demonstration for Inter-Data Center Transmission over 100 km of SSMF at 1550 nm,” OFC 2016, which shows 8 channels of 50 Gbps PAM4 modulated signal transported over 100 km of Single Mode Fiber (SMF). A second approach introduces more Digital Signal Processing (DSP) complexity as described in L. Zhang et al., “C-band single wavelength 100-Gb/s IM-DD transmission over 80 km SMF without CD compensation using SSB-DMT,” OFC 2015, and demonstrates a single channel 100 Gbps over 80 km of SMF using Discrete Multitone Modulation. A third approach, J. Wei et al., “100-Gb/s Hybrid Multiband CAP/QAM Signal Transmission Over a Single Wavelength,” J Lightwave Techn., 2015, uses 16-Carrierless Amplitude Phase (CAP) modulation and 16-Quadrature Amplitude Modulation (QAM) to deliver a single 100 Gbps channel over 15 km of SMF. Finally, a fourth approach described in A. Tatarczak et al., “Enabling 4-lane based 400 G client-side transmission links with MultiCAP modulation,” also uses a variant of CAP modulation to deliver 100 Gbps channel over 40 km of SMF.
An issue with all of the above approaches is that they sacrifice Optical Signal-to-Noise Ratio (OSNR) tolerance, which limits link margins, optical fiber reach, and demands higher laser optical power. These approaches also have very limited intrinsic chromatic dispersion tolerance, which frequently necessitates external optical dispersion compensation modules.
Therefore, practical industrial applications have focused on more complex fully coherent implementations that provide excellent noise tolerance and full electronic chromatic dispersion compensation. Proposed implementations are discussed as part of the 400 ZR standard in the Optical Internetworking Forum (OIF), as described in Weber and Williams, “400 ZR Modulation options,” OIF2017.049.01, January 2017. This implementation relies on a nested I/Q modulator to generate a 16-QAM constellation, and a coherent receiver with a 90-degree optical hybrid front end, and Analog-Digital Converter (ADC)+DSP complex to perform signal recovery and chromatic dispersion compensation in the digital domain. This approach is well understood and tested in the industry but requires extremely expensive deep nm Complementary Metal-Oxide Semiconductor (CMOS) process for ADC+DSP implementation to meet target power budget.
The primary shortcoming of current Intradyne Coherent implementations is high power associated with high-resolution, high sampling rate ADC. Second, complex DSP algorithms are required for extracting frequency, phase, polarization, and clock recovery. Therefore, deep nm CMOS nodes are required such as 7 nm, which results in extremely high Non-Recurring Engineering (NRE) and production costs. Further, narrow linewidth laser is needed to realize required phase/frequency stability, again increasing cost and complexity.